Java I/O and Exceptions

Transcription

1 Java I/O and Exceptions CS1316: Representing Structure and Behavior Writing to a Text File We have to create a stream that allows us access to a file. We re going to want to write strings to it. We re going to have to handle things going wrong exceptional events like the filename being wrong or the disk failing. Here s how Input and Output Streams - java.io Java handles input and output through sequential streams of bits Programs can read from a stream or write to a stream File String Source or Destination Array Byte Data Character Data Program Standard Input and Output We have been using System.out.println to print output to a PrintStream (standard output). System.out.println( First Name: + firstname); There is also a System.err PrintStream that can be used to write to the standard error output. System.err.println( Error: no file name given ); You can use System.in to read a byte or bytes from an InputStream (standard input). int numgrades = System.in.read(); 1

2 Chaining Input and Output Classes Often input or output classes are chained Passing one type of input/output class to the constructor for another One common thing is to chain a processing class with a data sink class Like a BufferedReader or BufferedWriter and a FileReader or FileWriter new BufferedReader(new FileReader(fileName)); Exceptions Exceptions are disruptions in the normal flow of a program. Exception is short for exceptional event. The programmer is required to handle checked exceptions in Java like trying to read from a file that doesn t exist Run-time exceptions do not have to be handled by the programmer like trying to invoke a method on a object reference that is null Children of RuntimeException Try and Catch Use a try & catch clause to catch an exception code that can cause exceptions catch (ExceptionClassName varname) { catch (ExceptionClassName varname) { You can catch several exceptions Make the most general one last All exceptions are children of the class Exception Try and Catch: If the file isn t there What if you want to know if a file isn t found That you are trying to read from If this occurs you might want to use a JFileChooser to let the user pick the file You also want to handle any other error code that can cause the exception catch (FileNotFoundException ex) { code to handle when the file isn t found catch (Exception ex) { 2

3 Catching Exceptions A catch clause will catch the given Exception class and any subclasses of it. So to catch all exceptions use: You can print the error message code that can throw the exception and the stack catch (Exception e) { trace (the list of all System.err.println( Exception: + e.getmessage()); currently running System.err.println( Stack Trace is: ); methods) e.printstacktrace(); You can create your own exceptions by subclassing Exception or a child of Exception. The optional finally clause A try and catch statement can have a finally clause Which will always be executed Will happen if no exceptions Will happen even if exceptions occur code that can cause the exception catch (FileNotFoundException ex) { code to handle when the file isn t found finally { code to always be executed Writing to a File Use a try-catch clause to catch exceptions Create a buffered writer from a file writer writer = new BufferedWriter(new FileWriter(fileName)); Write the data writer.write(data); Close the buffered writer writer.close(); Reading Lines of Character Data Enclose the code in a try and catch clause Catch FileNotFoundException if the file doesn t exist And you may want to give the user a chance to specify a new file Catch Exception to handle all other errors Create a buffered reader from a file reader for more efficient reading File names are relative to the current directory Loop reading lines from the buffered reader until the line is null Do something with the data Close the buffered reader 3

4 Reading from File Example BufferedReader reader = null; String line = null; // try to read the file // create the buffered reader reader = new BufferedReader(new FileReader(fileName)); // loop reading lines till the line is null (end of file) while ((line = reader.readline())!= null) { // do something with the line // close the buffered reader reader.close(); catch (Exception ex) { // handle exception Adding an Output File to WolfDeerSimulation /* A BufferedWriter for writing to */ public BufferedWriter output; /** * Constructor to set output to null **/ public WolfDeerSimulation() { Opening the File /** * Open the input file and set the BufferedWriter to speak to it. **/ public void openfile(string filename){ // Try to open the file // create a writer output = new BufferedWriter(new FileWriter(filename)); catch (Exception ex) { System.out.println("Trouble opening the file " + filename); // If any problem, make it null again Changing the time loop // Let's figure out where we stand... System.out.println(">>> Timestep: "+t); System.out.println("Wolves left: "+wolves.getnext().count()); System.out.println("Deer left: "+deer.getnext().count()); // If we have an open file, write the counts to it if (output!= null) { // Try it try{ output.write(wolves.getnext().count()+"\t"+deer.getnext().count()); output.newline(); catch (Exception ex) { System.out.println("Couldn't write the data!"); System.out.println(ex.getMessage()); // Make output null so that we don't keep trying 4

5 After the timing loop // If we have an open file, close it and null the variable if (output!= null){ try{ output.close(); catch (Exception ex) {System.out.println("Something went wrong closing the file"); finally { // No matter what, mark the file as not-there Running the Simulation with a File Welcome to DrJava. > WolfDeerSimulation wds = new WolfDeerSimulation(); > wds.openfile("d:/cs1316/wds-run1.txt") > wds.run(); Finding the file in Excel Adding Labels for the Chart Wolves Deer